The invention relates to a measuring device having a measuring slit with measuring zones, for measuring the characteristics of a moving thread-type test sample, which are associated with a measuring device.
Such a measuring device is known for yarn, for example from U.S. Pat. No. 3,377,852. In this meaning device, two electrodes are installed in a slit in a plastics block, in such a way that together they form a measuring capacitor, the measuring field of which crosses the slit. The surface of this slit is coated with a thin layer of a material, the electrical conductivity of which is less than that of the electrodes. This is intended to ensure that locally occurring electrostatic charges arising as a result of contact with the moving yarn are distributed over this layer and dissipated.
A disadvantage of such a known measuring device is that defining the conductivity of the thin layer is very difficult, since it is necessary to prevent the layer from effecting an expansion of the surface area of the electrodes. Moreover, it only makes sense to provide this layer if the yarn to be measured is electrostatically charged and frequently touches or might touch the side walls of the slit. In addition, this layer may become worn, which also means that the electrical properties of the slit then change again.
It is further known that optically operating devices are also available for measuring yarn, which likewise form a slit for the yarn. The beam path of an optical system extends over this slit, which optical system opens into the side walls of the slit.
Measuring slits in optically operating yarn measuring devices may be soiled by the yarn to be measured, which impairs optical measurement. This soiling may be counteracted by a degree of self-cleaning by the moving yarn. In order effectively to combat soiling by self-cleaning, the dimensions of the measuring slit must be favourably adjusted. A disadvantage of such an arrangement is that, in the case of narrow measuring slits, undue wear of the side faces or of the elements installed therein has to be expected. However, this restricts freedom of design in relation to such measuring slits and yarn measuring means in general.
The object to be achieved by the invention consists in increasing design options in the case of devices for measuring thread-type test pieces, in particular with regard to the construction of the measuring slit.
This is achieved according to the invention in that at least part of the measuring slit is covered with an abrasion-resistant coating, which is insensitive to wear by the moving yarn. This coating preferably covers electrodes, lenses or windows of optical devices embedded in the side walls of the measuring slit or parts thereof, as well as spaces or joints between the above-mentioned devices or electrodes and the other parts of the measuring slit. The measuring slit is coated by printing, dipping, vapour deposition, sputtering or spraying with a material which preferably enters into chemical combination with the surface of the above-mentioned parts in the slit and remains applied in a layer thickness of for example 20-30 nm.
The advantages achievable thereby are in particular that the service life of the measuring slit may be increased. Another advantage consists in the fact that the measuring field, i.e. the space in which there extends the beam path of an optical system or the electrical field of a capacitive system, may be reduced. This may be achieved on the one hand by a reduced slit width and on the other hand by a smaller surface area of the electrodes or the optical elements which adjoin the measuring slit. A further advantage consists in the fact that the self-cleaning effect of the measuring slit may be better exploited. This may be achieved by a reduced slit width. Soiling or deposits may then be more reliably removed by the test piece itself. This action is the greater, the narrower is the measuring slit and the more probable is contact between the test piece or protruding parts thereof and the side walls. Or, it is possible to dispense with lateral guidance, if the position of the test piece in the slit is of no importance.
A narrower slit also has the advantage that the effect of the shape, i.e. the fact that the cross section of the test piece may not be circular but possibly oval, on measurement of the mass of the test piece is reduced considerably. This is because, in a narrow slit, the yarn no longer appears to the electrodes or the optical elements as a flat body, as in a very wide slit.
The design according to the invention of a measuring slit also creates better conditions for installing an optical and a capacitive measuring system together in a measuring slit.